Structures and stabilities of H₂ adlayers on ionic crystal surfaces

Abstract

Our Monte Carlo (MC) simulations investigate the possible formation of a series of structures, p (2{604}2)[arrow right] p (8{604}2)[arrow right] p (4{604}2)[arrow right] p (6{604}2) with coverages |=0.5, 0.625, 0.75 and 0.83 respectively, by H 2 molecules on the MgO(001) and LiF(001) surfaces. On H 2 /MgO(001) the sequence p (2{604}2)[arrow right] p (4{604}2)[arrow right] p (6{604}2) is possible and is stable up to 9 K, while on H 2 /LiF(001) the progression p (2{604}2)[arrow right] p (8{604}2)[arrow right] p (4{604}2) is found. These structures are consistent with recent Helium Atom Scattering (HAS) results in terms of coverage and thermal stability, but disagree in terms of symmetry. The HAS work found " c " type structures whereas the MC simulations (without quantum considerations) yield a " p " type structures. The p (2{604}2) structures contains two H 2 molecules per unit cell, with each molecule lying parallel to the plane of the surface ([straight theta] = 90{493}) directly above every other cationic site. The molecules adopt a "T" configuration with respect to their nearest neighbours. For the p (8{604}2), p (4{604}2) and p (6{604}2) structures, there are two kinds of adsorption sites: a parallel site, as in the case of p (2{604}2) structure, and a tilted site, where the H 2 molecules sit between cationic and anionic sites with the molecular axis directed towards the anionic site, with [straight theta] {598} 60{493}. To reconcile the results of the simulations and experiments, the quantum mechanical rotational motion of the adsorbed H 2 molecules was studied using perturbation theory. These calculations show that the adsorbed hydrogen molecules are azimuthally delocalized and hence these structures are indeed c -type. For the H 2 /NaCI(001) system, the MC simulations of the bi-layer system show that the formation of a second layer is possible, in which the molecules lie tilted ([straight theta] = 45{493}) at the top of the Cl - sites in a unit cell of p (2{604}1) symmetry. It also found the molecules in the bottom layer lie flat over the Na + sites and arranged in a unit cell of p (2{604}1) symmetry, where the molecules adopt a tee configuration along diagonals, and have the same orientation along the rows. In terms of thermal stability, the bottom layer is stable up to 12 K whereas the upper layer is orientationally disordered at T > 5 K and molecules desorb at T > 8 K. These predicted structures are in agreement with the experimental findings in terms of coverage and thermal stability but disagree in terms of symmetry since the PIRS-ATR and HAS experimental findings show a (1{604}1) structure. To solve this problem, the rotational motion of H 2 molecules has been studied using perturbation theory and it is found that quantum effects will azimuthally delocalize the orientation of the molecular axis of H 2 . Thus, those predicted structures become a (1{604}1) structure.